Commercial fishing is an activity that is still dependent to a very large extent on the skill and intuition of the individual fisherman.
Very successful fishermen will always try to ‘think’ like a fish and try to imagine where the fish would prefer to be. It is generally acknowledged that good predictions can often be made as to where to commence fishing from observing the habitats of the different species of fish, and noting historically where fish have been caught in the past and in what numbers.
In practice, however, there is also much “throwing and hoping”, as well as a lot of fisherman's intuition.
Consequently, many methods have been developed to bring a more scientific approach to fishing, but these have principally been related to methods of detecting the presence of fish in a given location or the preferred habitat for the particular fish being sought. These methods have predominantly used acoustic echo sounders and other reflective “wave” techniques as the means for detecting fish and/or their habitat.
Whilst these acoustic sounders, or echo sounders as they are more commonly referred to, can detect the preferred habitat for bottom-dwelling fish life, such as Western Australian rock lobster, they are unable to actually detect such fish life on the sea or lake bottom. The reason for this is that these fish appear to be part of the sea or lake bed itself, at least in so far as the acoustic sounder is concerned.
Acoustic sounders, however, are able to detect the physical presence of middle and top-dwelling fish life. However this only occurs when the vessel containing the acoustic sounder is virtually right on top of the fish.
Thus, in general terms, acoustic sounders may at best be described as fish detectors, but certainly not fish predictors.
Sometimes top-dwelling fish can often be detected by other natural means, such as bird activity, or alternatively, visual observation of surface water activity. Mostly, however, for these types of fish, surface fishing gear has to be set up on an ad hoc basis to see if fish are present in that area.
Whilst these techniques are effective in showing the quantity in species of mid-dwelling fish in a particular area and the likely habitat for bottom-dwelling fish, they do not detect the water conditions or the environment in that area. In this respect, it is believed that water conditions are more important than habitat to a fish, and in fact fish will follow water conditions that are favourable to them. Further, it is believed that certain types or species of fish do not like sudden changes in depth, water temperature, salinity and certain other environmental aspects, but they prefer to acclimatise slowly.
Contrary to popular belief, it has been established that the water in the seas, oceans, rivers and lakes of the world are not one homogenous mixture, but rather are a collection of very different patches of water. Whilst the temperature obviously varies according to the whereabouts in the world that the body of water in question is located, it has also been discovered that the acidity, dissolved oxygen, salinity, turbidity and ORP (oxygen reduction potential) also varies significantly.
Significant variations can occur within quite close distances to one another. For example, there are known places in oceans and seas where there are fresh water springs where there is obviously low salinity proximate to and downstream or down-current of the spring and high levels of salinity at places similarly proximate to but upstream or up-current of the spring. Indeed, the change in salinity is a factor experienced proximate to a river mouth, where differences in salinity can vary considerably dependent upon the season and rainfall.
It is believed that fish, like most other living species prefer to live in the most comfortable habitat and environment that they can find. However, if that habitat and/or environment should commence to degrade, the fish will move, if possible, to a better one.
Whilst habitat can change, generally it does so over long time periods. On the other hand, environmental changes may occur very quickly. An example of this is in the ocean where certain types of seaweeds can break free and then quickly decompose. The products of decomposition can include ammonia, methane, iodine, protein and hydrogen sulphide. amongst others. These products can rapidly affect the immediate environment proximate to the decomposing seaweed.
Whereas some species of fish, such as Western Australian bream or snapper can be very tolerant of a wide range of ammonia, they find even minute levels of sulphuric acid (as produced by the hydrogen sulphide dissolving in water to form sulphurous acid, combining then with the dissolved oxygen to form sulphuric acid) to be very toxic.
On the other hand, crustaceans such as crabs and lobsters are very intolerant of small amounts of ammonia, but can live in a wide range of ORP.
Yet again, tuna is very sensitive to temperature and iodine, but is insensitive to ammonia.
When the fish find this change in the environment uncomfortable, they generally commence to move to a better one.
Similarly, a temperature of a sector of ocean can not only change significantly from one locality to another, but also from one depth to another. For instance, under certain specific conditions, it is possible to have warmer water underneath colder water.
The temperature of the water in which the fish are living is therefore an important parameter, so that in the case of fish living at, or near, the seabed, the temperature of the water at that depth is much more important than the surface water temperature. Conversely, for top-dwelling fish, the water surface temperature or the thermocline up to 6 metres deep is believed to be the more important parameter.